1. Field of the Invention
The invention relates to an arrangement in an apparatus for forming a windscreen interlayer.
2. Description of the Background Art
The polyvinyl butyral films used in the manufacture of screen glass are worked in a continuous process by heat treating films reeled off rolls. The heat treatment is carried out as radiation heat in a multi-lamp radiation unit. A temperature difference is created in the radiation unit between the different margin areas of the web in the cross direction of the web. The hot margin area is typically heated to a temperature of about 90.degree. C. whereas the cold margin area is typically maintained at 30 to 50.degree. C. By providing stretching in the heat treated material web, the curvatures needed in screen glass are obtained. The curvature is controlled by means of both the temperature and the stretching.
Today, a shaded film is increasingly often used as an anti-dazzle barrier at the brim of the film. During the heat treatment, this dark brim is the cool section of the web. As the shaded portion of the web absorbs a greater portion of the radiation than the clear section, the excess temperature rise constitutes a problem during the heating process.
The problem has been approached by putting out radiation sources at the cool brim of the web and by placing at the dark brim a radiation-barrier board which is aligned parallel with the plane of web. In the case of new shading materials, sufficiently good process control properties have not been attained by this method, and it has not been possible to form the desired temperature distribution in the web.